Prediction in physics

Prediction in physics

A ball projected in the air moves in accordance with the laws of gravity, but the particular path it follows depends very much on the speed and angle of projection. Calculating physicists can predict the precise spot where a missile will land when it is fired in a particular way from a particular place or the precise time when a comet will reappear in the skies. This is because the world evolves in accordance with immutable physical laws, and the specific modes by which its countless atoms and molecules move and interact were determined by the initial kick that each received.

This is predestination in physics. This was the picture painted by physicists of the eighteenth and nineteenth centuries who, on the basis of their understanding of physical laws cast in differential equations, believed that the primordial push to the evolving universe determined once and for all times how every single atom and molecule would behave at every instant in the future. Laplace, the most eloquent exponent of this view, stated: “We ought then to regard the present state of the universe as the effect of its anterior state and as the cause of the one which is to follow. Given for one instant an intelligence which could comprehend all the forces by which nature is animated and the respective situation of the beings who compose it – an intelligence sufficiently vast to submit these data to analysis – it would embrace in the same formula the movements of the greatest bodies of the universe and those of the lightest atom; for it, nothing would be uncertain and the future, as the past, would be present to its eyes” In other words, every aspect of the universe that is to emerge could be foreseen by a calculating super-intelligence that could track down the dynamic states of the constituents of the world.

Such a view does away with time as a significant feature of the physical world, for the occurrence of phenomena is simply like the turning of the pages of a book here, as suggested by Islamic visions, everything is already inscribed. Time does not play a part in the evolution of phenomena, it is a static course on which things appear to be happening. In this view, events in the world are like images on the walls of a long tunnel through which the train of consciousness is moving. Each of us, a passenger in the train, zooms past the images, experiencing and recording the scenes that come our way. In the process we feel as if the scenes are changing (i.e. that time is flowing) when, in fact it is the conscious spirit that is hurtling forward.

There is another sense in which physicists predict. Theoretical physicists can sometimes unveil the existence of entities or phenomena in the physical world without making any direct observations themselves, and of which no one had any inkling before. Consider the following: For many centuries humankind had recognized only seven heavenly bodies moving somewhat differently than the stars: the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn.

A new planet was discovered in the 1780s by the astronomer William Herschel. It came to be called Uranus. The motion of Uranus was studied painstakingly by astronomers for the next 50 years, during which time certain discrepancies were discovered: It did not seem to be following the smooth path that a self-respecting planet was expected to take. By the 1840s this peculiar behavior of Uranus was explained by the mathematical astronomer John Couch Adams and the chemist-turned-astronomer Urbain Le Verrier. By applying sophisticated mathematics to the problem, both of them independently came to the same conclusion.

“According to my calculations,” wrote Adams on October 21, 1845 in a letter to the Astronomer Royal, George Airy, “the observed irregularities in the motion of Uranus may be accounted for by supposing the existence of an exterior planet.” On September 18, 1846, Le Verrier wrote in a letter to Johann Galle, another astronomer, “It is impossible to satisfy the observations of Uranus without introducing the action of a new planet.” In about a week’s time such a planet was indeed discovered, very close to the location that had been predicted by the calculations.

A remote astronomical body’s existence and position were discovered on paper on the basis of calculations, using the laws of physics and the knowledge of behavior of another planet! It came to be called Neptune. This was a truly impressive achievement of the human spirit, no less spectacular than our technological wonders. And it is a fine example of prediction in science.

Whereas predictions in non-science are qualitative and related to human events, those in science are quantitative and invariably refer to phenomena in the physical world which are generally indifferent to human concerns.